Scafold materials are prepared from native tissues for clinical application by chemically treating the tissue with a fixing agent such as glutaraldehyde or by decellularizing the tissue. In heart valve replacement, for example, xenogeneic heart valves are prepared from porcine heart valves or bovine pericardia by treatment with glutaraldehyde to diminish their immunogenicity. These xenogeneic valves are highly anti-clotting but durable only for 5-10 years in young recipients. Therefore, they are normally transplanted to older recipients over 60 years old.
Since tissue bank systems have been organized in Europe and America around 1985 and also in Japan in recent years, allogeneic cryopreserved valves from deceased donors have been clinically used. The allogeneic valves are less thrombogenic than mechanical valves, more durable than xenogeneic valves and less susceptible to infections than both. However, a critical problem is the fact that the number of available valves is absolutely insufficient. Moreover, cases in which functional failure appeared at a relatively early stage have been reported among young recipients suggesting the involvement of immune reactions. In the Ross operation know to be effective in young recipients, autologous pulmonary valve is transplanted to aortic valve site and the impaired pulmonary valve is reconstructed with cryopreserved allogeneic valve. The characteristic feature of the autologous pulmonary valve transplanted to the aortic valve site is that it is growable as the recipient grows. In contrast, mechanical valves and xenogeneic valves as well as cryopreserved allogeneic valves cannot grow and re-transplatation is often needed for children. In order to eliminate the above problems, several studies have been reported removing donor cells from allogeneic valves so that their immunogenicity and involvement of immune reactions are diminished to increase the durability and autogenesis of transplanted valves.
A decellularization method using a chemical solution called “SynerGraft” was developed by CryoLife, U.S.A. It was reported that the decellularized tissue by this method was infiltrated into autologous cellular structures within several months and recellularized with autologous cells.
Harverich et al. of Hannover University, School of Medicine, Germany published a decellularization method using the detergent Triton X-100 and proteolytic enzyme tripsin solutions.
However, washing with detergent or other chemical solutions alone is not sufficiently effective to remove bacteria, viruses and other contaminants from the interior of tissue because the washing depends on diffusion and penetration of the washing solution from surfaces of the tissue. Because of these limitations, complete decellularization and removal of bacteria and viruses are hardly possible for large tissue materials. In order to achieve satisfactory effects by chemical washing, it is necessary to increase the degree of treatment. This may lead to problems of post-graft calcification and removal of residual treating chemicals. As evidenced from BSE and CJD infections in transplantation, safety assurance is very important for the tissue to be transplanted. Currently known treating processes do not assure complete inactivation of viral contaminants and infection incidents may often occur from transplanted tissue contaminated with viruses.
The decellularized xenogeneic or allogeneic tissues are recellularized by seeding and culturing autologous cells for transplantation as a hybrid regenerative tissue.